1. Technical Field
The present invention relates generally to electronic input devices, and more specifically to trackpads and finger-tracking devices.
2. Background Art
A variety of input devices are used with modem computers. As computers have become more complex, so too has the choice of input devices grown. Certain input devices are more suitable for given tasks, or for use with a given application or computer system. For example, keyboards are typically used for text input, while mice may be used for object selection and/or browsing.
Trackpads are stationary pointing devices often used where space is at a premium, such as with laptop or notebook computers. Trackpads (also referred to as “touchpads”) typically provide a small, flat surface over which a user may slide a finger to move a cursor or pointer on a computer screen. In this manner, trackpads provide functionality similar to computer mice. Further, the computing system may equate tapping the trackpad with a finger to clicking a mouse button. Thus, a trackpad may be seen as an equivalent device to a mouse.
Many trackpads couple capacitance through a finger (or other conductive pointer) to operate. In simplest terms, a trackpad includes two layers of electrodes arranged to form a grid, as shown in FIG. 1. For example, the first layer may include horizontally-aligned, strip-shaped electrodes and the second layer may include vertically-aligned, strip-shaped electrodes. The layers are positioned such that one layer overlies the other to form the aforementioned grid. Each of the electrodes is connected to an integrated circuit, for example. The integrated circuit is typically (although not necessarily) located beneath the electrode grid.
As a finger moves across the grid, the finger forms the second plate of a two-plate capacitor. The finger effectively capacitvely couples to the lower electrode, thus changing the capacitance of the electrode beneath the finger by some small amount. In some cases, the change in capacitance is on the order of 1-2%. The integrated circuit (or a computer system associated with the trackpad) continuously measures the capacitance of each electrode in the grid. When the integrated circuit detects an increased capacitance in the electrode at a certain spot of the grid, the position of the finger is located. The integrated circuit may, for example, continuously scan each row and column of the electrode array (grid) to measure capacitances. Alternative trackpads may employ only a single layer of electrodes (i.e., only rows or only columns). The operations described herein, including coupling of the finger to the electrode, generally operate in a similar manner. It should also be noted that multiple fingers, or other capacitvely-coupled elements, may be tracked by a single trackpad.
The computing system may use the finger's absolute location on the trackpad to create relative pointer data, thus placing or moving a pointer on a screen associated with the computing system. Similarly, if the integrated circuit detects a finger's presence at one time, no presence at a second time, and the finger's presence in approximately the same area at a third time, the computing system may interpret this as a tap on the trackpad analogous to clicking a mouse button.
Many trackpads, especially those used in notebook computers, are shielded from direct contact with a finger by an insulator layer. For example, aesthetics often demand a metallic look for notebook computers. Such a look may be accomplished either by plating the laptop with metal, or applying metallic flake in a composite, plastic, or paint layer to the laptop. Typically, the metallic finish is an integral part of the laptop body to prevent cracking or peeling with age and/or use. However, many times the metals used to enhance the laptop's appearance act as an insulator between a user's finger and the trackpad. This is especially true where the trackpad surface is finished with such a metallic layer. The presence of this insulation layer may decrease the signal-to-noise ratio of the trackpad (and thus, the trackpad's sensitivity) by up to 60%. In many applications, a mylar label having paint on its backside may be adhered to the trackpad in place of the metallic finish. This paint may have a metallic finish. Similar problems to those described above are presented in such applications.
Additionally, more sensitive trackpads may enhance a user's computing experience by providing more precise input. By providing more precise input and enhancing the range of fingers that may capacitvely couple to the trackpad, a wide range of applications may be opened.
Many present solutions attempt to increase the sensitivity of a trackpad by employing electrodes having a larger surface area in the aforementioned electrode grid. However, a linear relationship exists between a parasitic capacitance experienced by an electrode and the area of the electrode. Thus, as the electrodes used in the electrode array increase in size, so do the parasitic capacitances associated with the array. A parasitic capacitance generally decreases the trackpad sensitivity by decreasing the maximum change in capacitance that may be experienced between the upper and lower electrodes. Further, at some point the parasitic capacitance may be greater than the change in capacitance experienced at a point on the electrode array when the finger touches the trackpad. Thus, if electrodes having a sufficiently large surface area are used, the trackpad may not be able to detect when a finger is placed against it.
Accordingly, there is a need in the art for a trackpad having an improved capacitive sensitivity.